1. Field of the Invention
The present invention relates to a method for producing a thermoplastic resin molded article or, in particular, a fiber-reinforced thermoplastic resin molded article whose one surface is laminated with a skin material, and a mold assembly used for the method.
2. Related Background Art
Since porous fiber-reinforced thermoplastic resin molded articles are lightweight and excellent in terms of strength, they are widely used for interior automotive parts (e.g., a door trim, a rear trim, and an instrument panel), interior and exterior parts of home electronic equipments, building materials, and the like. Conventionally, as a method for producing such a porous fiber-reinforced thermoplastic resin molded article, a so-called expansion molding method has been known. According to this expansion molding method, initially, a stampable sheet, i.e., a fiber-reinforced thermoplastic resin sheet, is preheated so as to be expanded in the thickness direction thereof. Thereafter, thus expanded sheet is supplied to between upper and lower molds while maintaining the preheated state, pressed between the molds, and then cooled. In this case, the closing clearance between the upper and lower molds is set to a value which is greater than the thickness of the fiber-reinforced thermoplastic resin sheet before preheating but smaller than the thickness of the expanded sheet. Thus, a porous fiber-reinforced thermoplastic resin molded article having a thickness greater than the thickness of the fiber-reinforced thermoplastic resin sheet before preheating is obtained.
The surface of the porous fiber-reinforced thermoplastic resin molded article produced by such an expansion molding method tends to be embossed with its reinforcing fiber from the inside, thereby deteriorating its outer appearance and feel. Accordingly, when a thermoplastic resin molded article is used as interior automotive parts, it is generally used with its surface being laminated with a skin material. As a method of laminating a skin material on the surface of the thermoplastic resin molded article, there has been known a method in which, a step of making the skin material and the fiber-reinforced thermoplastic resin sheet, which has been expanded by preheating, overlap with each other is added to the above-mentioned expansion molding method, whereby thus overlapping materials are pressed between the upper and lower molds and laminated together (Japanese Patent Application Laid-Open Gazette No. Hei 4-331137 (331137/92)). In this case; the closing clearance between the upper and lower molds is set to a value which is greater than the sum of the thickness of the fiber-reinforced thermoplastic resin sheet before preheating and the thickness of the skin material but smaller than the sum of the thickness of the sheet after expansion and the thickness of the skin material.
According to this method, the skin material follows the inner shape of the mold so as to reproduce the mold shape on its surface. In practice, however, there is a tendency that the mold shape may not be faithfully reproduced on the surface of the skin material as a designed surface, the surface may not become a desired shape, and the contour of the surface shape may become unclear.